1. Field of the Invention
This invention relates to plasma chamber tuners. More particularly, it relates to a solid state tuner used to measure and match voltage standing wave ratios between an RF generator and a plasma chamber.
2. Description of Prior Art
Plasma chambers are low pressure, gas filled enclosures used in processing operations such as RF sputtering, plasma etching/depositions, and reactive ion etching. These processing operations are used primarily for integrated circuit fabrication. The plasma chambers are excited by RF generators running generally in the ISM frequency bands of 13.56 MHz, 27.12 MHz, and 40.68 MHz. Although lower and higher frequencies can be employed with plasma processing systems, 13.56 MHz is the predominate frequency used in the RF generator market. Typical power levels for the RF generators are in the range of 250 to 2500 watts and usually designed employing transistors operating in saturated class AB or D.
The design of the RF generators make them sensitive to voltage standing wave ratios (VSWR) of the load in a plasma processing system. Regulation of the forward power and thermal derating is necessary as well as protection over a wide changing impedance range. Further, since the process being performed by the plasma chamber is sensitive to the power delivered, wherein fluctuations can result in poor processing quality by the chamber, control of any fluctuation in power due to the interaction between the RF generator and plasma chamber is required.
Many of the regulation, protection, and control requirements, due to the interaction between RF generators and plasma chambers in a plasma processing system, have been addressed by inserting an automatic tuner between the generator and chamber. This has been done in the prior art by utilizing electro-mechanical tuners. Mostly, the prior art tuners are motor driven devices employing variable capacitors for adapting the output impedance of the generator to the input impedance of the chamber.
Although many of the electo-mechanical tuners of the prior art have been successful in matching a wide range of VSWR's, they possess many inherent deficiencies. Examples of these deficiencies include: slow response time--typically one to two seconds; poor reliability--mean time between failure (MTBF) figures are the lowest of any component used in the processing system; loss--the tuner can have a significant power loss during its generator to chamber impedance matching; varying loss--causes power to the chamber to fluctuate resulting in poor processing within the chamber and requiring a higher powered RF generator; additional cooling requirements are usually needed for both the RF generator and tuner because of the loss; expense--electro-mechanical tuners can cost as much as the RF generator, a cost most likely passed on to the consumer by the operator or owner of the processing system.
There exists a need for an improvement to tuners used in plasma processing systems to eliminate the inherent deficiencies encountered with tuners of the prior art.